1. Field of the Invention
The invention relates to a set of prefabricated molars for a set of removable full dentures.
In the dental field, i.e. the technical dental field, prefabricated produced molars are being used with complete full dentures, hybrid dentures and partial dentures and are applied by persons, who are missing some, or all of their teeth. These prefabricated produced molars are being secured and become part of the denture, and together with its synthetic material, forms a solid unit. Such dentures are quite different from crowns and bridges, where the individual molars will be reconstructed completely, meaning they are being constructed in a dental laboratory and will be cemented directly to the previously prepared stump of a tooth.
Hybrid dentures completely cover structures that are located underneath, making securing the dentures to the stumps of the teeth, and through recently invented prefabricated implanted roots (implants) possible, by implementing a push-button-like system. They are, however, with possible exceptions, basically constructed like a normal removable denture, which has prefabricated molars attached to its base.
2. Description of the Prior Art
In the past century, prefabricated molars were made of porcelain and placed in dentures, which had no consequent and specified form relating to their function. Beginning in 1908, A. Gysi developed prefabricated molars, which were strictly constructed according to functional principles. The result of the development currently consists of three different construction types of prefabricated molars, with each clearly defining its functionality, which was actually produced and is still being utilized in the dental field i.e. technical dental field to date. The individual molars are securely anchored in the framework of the denture and have functional contact with the opposing teeth. Each tooth influences the stability of the entire denture and the other teeth of the same denture, since all teeth are securely connected to the framework of the denture. It is therefore important to pay attention to the impact of each individual molar when under the pressure of chewing, as well as the functional impact of all molars together, when constructing the denture. If a denture is to be made at the same time for both, the maxilla and for the mandible, the impact of the reciprocal functionality of the individual molar pairs, as well as all molars together, must be observed.
With "classical" defined type 1 construction (according to A. Gysi; for example compare patents CH-109795, CH-199038 and CH-88645), the molars are equivalent to the shape of the human molars, with perhaps some deviations. They however have no roots but only show the crown of the tooth, which is the part that protrudes from the gum and is visible with natural teeth. In the following description, the term "cusp" or "dental cusp" refers to the conical shaped pointed or rounded eminence on or near the masticating surface of the tooth which occludes with the antagonizing tooth of the opposite dental arch, and the term "fossa" refers to a shallow depression concavity or a hollow depressed area. The oral/palatal cusps of the upper molars which face the tongue, bite in the direction of a central fossa of the lower molars and/or a dental cusp in contact with two opposing teeth, which form central fossa when put together. The buccal cusp of the lower molars, which point toward the cheeks, fit into the central fossa of an upper molar or in the central fossa which was formed by two upper molars and make contact with the outer buccal cusp of the upper molars. The fitting into each central fossa of all cusps is called "classical occlusion".
When moving the mandible, in relation to the maxilla, which is attached to the scull, the cusp of the upper molar forms, stabilized contacts in the shape of gliding surfaces and is therefore essential. According to A. Gysi, all molars of the upper denture shall remain in contact with the lower denture, in order to prevent tipping when the patient moves the mandible. This way the denture will always be supported by a large surface area on both sides (bilateral balancing).
The disadvantage of this type 1 construction is that the functional contact between the outer buccal cusp of the upper and the lower molars very often causes a tipping of the upper and/or the lower denture, when rough and hardly processed bolus is still present. In order to prevent this situation, the molars must be placed in the tongue area, in order for the outer pressure-bearing cusp to be positioned at least in the center of the carrying alveoli ridge, which will however restrict the tongue area considerably. The contact of the outer buccal cusp however may be eliminated, in order to shift the chewing forces inward lingual. The shape and the position of the premolars without bolus being present, makes it impossible or at least very difficult, to stabilize the gliding contact through movements to the side and partially by displacement movements of the mandible, as it is required for the bilateral balancing. Since the molars will be under functional pressure when chewing bolus, the stability of one-sided use is essential. Also between meals and while sleeping, intensive functional contact of the teeth without bolus, will occur.
Resolving the aforementioned disadvantages, molars have been developed according to a construction of type 2 (according to A. Gerber; for example compare patents CH-405601, U.S. Pat. No. 3,305,926 and CH-607686), where the bilateral balancing remains secured and the dentures remain stable and even on the base. That means that they are resting on the appropriate jawbone of the alveoli ridge. For stability reasons the palatal/oral inner upper cusp of the upper molars, which are pointed toward the tongue, are dominantly being used for chewing, in conjunction with the lower central fossa of a molar, or with a central fossa created by two molars. The cusp in this instance, is in general convex-shaped and the appropriate central fossa is in general concave-shaped, so that the molars (in perspective to the upright-positioned denture carrier) create a vertical line of force at a random slant. The outer buccal cusp of the upper molars however do not show any or only show a clearly reduced contact, which is very small and which is located closely near the upper central fossa. The center for chewing and the chewing force are clearly being shifted to the inside (oral), toward the center of the mouth and not to the outside of the upper and lower alveoli ridges, which means not outside the bone structure which carries the masticatory force; since the masticatory forces, which take place outside of the center of the upper and lower alveoli ridges, may cause the denture to tip. The upward outward pressure against the upper and slightly outward-tipped buccal cusp, of the upper molars will completely be deleted or will at least be drastically reduced. This way there is plenty of room for the tongue, without loss of stability by the masticatory force, compared to the construction of type 1. The molar-relief essentially complies with the natural model, however with partially clear deviations.
During a normal and one-sided chewing process, with the bolus being large and hard in the beginning, making the contact of the molars impossible, the outer buccal cusp are not being used. This is called "lingualized occlusion".
During sideways motions of the maxilla, without bolus being present, all molars remain in constant contact with one another (bilateral equilibration balancing), where only the inner palatal molars have contact with the opposing tooth. The upper outer cusp overlap the lower outer cusp, clearly without any contact, since they could make the denture tip when bolus is present.
These molars have the disadvantage, when type 2 construction has been applied that their chewing efficiency is smaller then the chewing efficiency of the molars of the construction type 1. The functionality of the convex-shaped cusp with the concave-shaped central fossa creates a vertical alignment of energies. This is contingent on the fact that the lower tooth must be positioned in this line of force and therefore restricts the space for the tongue.
Combination constructions of type 1 and type 2 are created, when for example a construction by H. P. Foser, which permits the classical occlusions position as in construction type 1, and the lingualized occlusion, like in constructive type 2, and a construction according to H. Schroder (compare for example patent CH-161975) for a molar, where the upper central fossa and the active lower cusps are integrated in the particular denture. This shape is non-anatomical, but is also not flat, like in construction type 3, which follows.
The basic principle of construction type 3 is not to integrate any obstructions for movements of the mandible through a flat chewing surface. The chewing surfaces of the lower molars and/or the upper molars no longer follow the natural molars (non-anatomical molars) but the particular chewing surfaces of such molars are flat. That way, the molars cannot be completely adjusted to the movements of the mandible and cannot be maintained in reciprocal (bilateral) contact. There will always be some loss of contact, whether it is on the inner or outer sides.
With anatomical molded prefabricated molars of the construction types 1 and 2, the oral inner cusp of the molars are situated in the opposite, antagonistic central fossa, in a functional occlusion, where the cusp may be in the molar of the upper denture as well as in the molar of the lower denture and where the appropriate central fossa is either located centered to the opposing tooth, or is formed through two molars, located next to one another. The formation of the cusp and the central fossa create a vector force through their opposing function.
In an ideal situation the central contact function of each set of molars (or also between three molars) of each upper and lower vector force of the occluded molars (left or right side of mouth), meets the center of the supporting alveolar ridge. The alveolar ridge where the natural molars were originally fixed, is the supporting base of the denture for a person without teeth. If the projection of the vector force of the denture from the top and from below, during the masticatory force, is outside the supporting jaw ride, the denture will tip when masticatory force is applied. This applies to an individual molar, as well as to all molars of the denture, when they are under pressure.
Seen from a frontal (transversal) view, the molars of the upper alveoli ridge, compared to the molars of the lower alveoli ridge, are in most cases positioned slightly inward off-center, depending on the degree of the resorption of the jawbone. In order to avoid tipping of the upper denture, the inner cusp or central fossa of the upper molars must ideally be situated vertically under the center of the alveoli ridge. Therefore, depending if the vector force runs vertically, the lower molars with their function center (central fossa or cusp) is located partially considerably within the alveoli ridge. This way, the tongue may be restricted in the area of the lower denture, which will bother the patient while speaking and chewing. To have more room for the tongue, it would be advantageous to divert the vertical vector force in the direction of the lower alveoli ridge, which is located more toward the outside. This means orienting the vector force along the so-called "interalveolar line", which is the imaginary connecting line from the center of the alveoli ridge on top, to the center of the alveoli ridge below, deviated by approximately 10.degree.-15.degree. from the vertical. The molar (in general the lower molar) may be placed out further, buccal toward the alveoli ridge, without effecting the stability of the upper or lower denture.
The side (sagittal) view indicates that the molars, depending on the shape of the alveoli ridge and the degree of resorption of the jaw bones, which carry the denture, stand above a slanted and forward tipping plain, with the higher alveoli ridge naturally rising upward toward the back. Loss of the alveoli ridge can increase this problem considerably. Through the masticatory force, the denture glides forward on this slanted plain. Only through a diversion of the line of force toward the lower back can the molars be made stable for chewing. Only when the vector force meets the mandible ride in the proper angle or at a point and toward the back-oriented angle, is a forward gliding of the lower denture prevented.
An optimal adjustment of the vector force requires type 1 construction, a "classical occlusion" with all of the all cusps fitting into a central fossa. This means, an occlusion of the cusp of the upper molar, pointing orally/palatal toward the tongue, and toward a corresponding central fossa of the lower molars and at the same time an occlusion of the cusp of the lower molars, facing the cheek, toward a corresponding central fossa of the upper molars. It is not possible to direct a given cusp of a given molar in the optimal direction, when the vector force produced by chewing in sideways direction toward the inside/lingual or outward/buccal, as well as in sagittal direction, toward the back/distal or toward the front/mesial, individually or as a combination, and in any case independent of other cusp of that molar. The necessary constructive spacing is missing. The displacement of the lower denture toward the front may be partially avoided by additionally sloping the molar until it is parallel with the supporting alveoli ridge. To achieve this, it is required that the outer buccal cusp are engaging with the lower cusp, otherwise with only the inner cusp occluded with the corresponding central fossa, a diversion of the vector force is not sufficient.
In comparison, construction type 2 does not lead to the correct orientation of the vector force, since the active convex-shaped cusp in the appropriate spherical occluding central fossa, creates a vertical line of force even though both molars are tipped. The outer buccal cusp may not be used for correction of the vector force, since they may not come into contact because they would jeopardize the transversal static.